IP THE CRANIUM, THEN MOVE THE LIGHT BACK AND FORTH over the molars.”

I did as Bergeron instructed.

“Do you see a glossiness in the folds of the enamel?”

I didn’t.

“Angle the beam.”

Bergeron was right. The shine was subtle but present, way down in the grooves.

“What is it?”

“If I’m not mistaken, the molars have been treated with a pit and fissure sealant.”

When I looked up, Bergeron was gangling his way to the scope. The man was definitely not poetry in motion.

“Sealant is a thin coating of plastic resin that’s applied to the chewing surface of a bicuspid or molar. It’s painted on as a liquid, and in roughly a minute it hardens to form a protective shield.”

“What’s the purpose?”

“To prevent occlusal caries. Tooth decay.”

Bergeron slipped the lower jaw of LSJML-38428 under the lens, peered through the eyepieces, and adjusted focus.

“Oui, madame. That’s a sealant.”

Hope did a little moth-flutter in my chest.

“When did these sealants come into use?”

“The first commercially available sealants were marketed to dentists in the early 1970s. They’ve been in widespread use since the eighties.” Bergeron spoke without looking up.

The moth exploded into a hummingbird.

The girl in the leather shroud couldn’t have died in the fifties! By elimination, that jumped her to the late eighties!

I tried to keep my voice calm.

“How common are these sealants?”

“Unfortunately for forensic purposes, very. Most pediatric dentists recommend application once the permanent molars erupt. School-based programs have been under way in a majority of American states for at least twenty years. Canada’s a bit behind in that, but sealants have been very popular here since the mid-eighties.”

Bergeron clicked off the fiber-optic light.

“Didn’t help this young lady much.” He thrust his chin at Dr. Energy’s girl. “She’s got more decay than that one over there.”

“So she was seeing a dentist at one point, then quit caring for her teeth.”

“Typical pattern for runaways. The parents provide dental care while they’re growing up, then the kids hit the streets, their diets and hygiene go to hell, and their teeth suffer.”

“How old was she?”

Bergeron returned to the light table and examined the dental X-rays for 38428.

“A little older than the others. I’d give her eighteen to twenty-one.”

Again, Bergeron’s estimate was consistent with what I’d seen in the bones.

“Any evidence of sealant on the other two?”

Bergeron reexamined the teeth of 38426 and 38427. Neither had been treated.

“A pity there are no restorations on any of them. Let me know if there’s anything else I can help you with.”

“You’ve helped plenty.”

I flew to my office and dialed Claudel.

He and Charbonneau were tied up in an interview and couldn’t be disturbed. I left a message requesting they call me as quickly as possible.

Returning to my lab, I picked up a fractured segment of jaw that Bergeron had left beside the scope. As I was returning it to LSJML-38427, I noticed a tiny nick on the right mandibular condyle.

Back to the scope.

By angling the fiber-optic light across the bone, I found two more nicks on the ascending ramus, and a minuscule groove at the mandibular angle.

I checked the left portion of the mandible.

No nicks or grooves.

The skull.

No nicks or grooves.

One by one I examined the isolated shards broken from the right cheek and temporal bones.

The light picked out six superficial grooves, each roughly five millimeters in length, grouped in three sets of two.

Another shoulder tap from my hindbrain.

I increased the magnification.

The nicks and grooves, though clearly not natural, looked different from those on 38428. Though V-shaped, these were much narrower in cross section and cleaner at the edges.

Like marks left by a scalpel. In fresh bone.

I leaned back, thinking through what that could mean.

In my mind I reconstructed the skull fragments and articulated the jaw.

The cuts circled the ear opening.

What the hell had gone on?

Coincidence? Something more sinister?

I was about to reexamine the skull and mandible of Dr. Energy’s girl when I spotted Charbonneau through the window over the sink. Gesturing him to my office, I stripped off my gloves, washed, and crossed the hall.

Charbonneau had assumed his usual legs-splayed, shoulder-slumped position in the chair facing my desk. Today’s jacket was cranberry and as glossy as the dental sealant.

“Monsieur Claudel is meeting with the Nobel committee this morning?”

Charbonneau dipped his chin, rolled his eyes up, and raised both palms.

“What? I’m not cool enough? Luc really is busy.”

“Being fitted for another Ermenegildo Zegna?”

Charbonneau looked at me as though I’d spoken Etruscan.

“They make suits,” I said.

Charbonneau suppressed a grin. “He’s going through Cyr’s tenant list.”

“Really?” My brows shot up in surprise.

“Authier phoned.”

LaManche must have spoken with the chief coroner, who then ordered Claudel to get serious about the pizza basement case.

“Not a lot of jolliness in Authier’s message?”

“Luc is viewing the comments as suggested guidelines.”

I explained Bergeron’s discovery.

“Bergeron’s convinced it’s this sealant stuff?”

“Absolutely. I believe that’s what journalists call independent corroboration.”

“So at least one of the three died in the seventies or later.”

“Carbon 14 analysis bracketed this girl’s death in the fifties or in the eighties.”

“Guess we’re talking the eighties.”

“Guess we are.”

“The kid with the broken wrist?”

I nodded. “The skeleton wrapped in the leather shroud.”

“Son of a bitch.” Charbonneau pushed to his feet. “I’ll get her stats into the system right away.”

Charbonneau had barely cleared the door when the phone rang. It was Art Holliday, calling from Florida.

“You got the Carbon 14 report?”

“Yes, thank you. I appreciate your turning it around so quickly.”

“We aim to please. Listen, I may have something else for you.”

I’d forgotten Holliday’s offer to perform additional testing.

“For prosecutorial purposes, strontium isotope analysis is still experimental. But we have applied the technique to forensic questions. In one case, we nailed the point of origin of six white-tail deer. Used the antlers. Course, we knew the animals had to have come from one of two states, so we had isotopically distinct geographic localities from which to measure control groups. That made the job easier.”

Over the years I’ve learned that it is impossible to hurry Art Holliday. You go with the flow, half listen to the buildup, and focus on the conclusions.

“We’re getting good results looking at immigration and settlement patterns with ancient populations.”

That rang an archaeological bell.

“Yours is the group analyzing the pueblo materials from Arizona?’

“Thirteenth- and fourteenth-century burials. Construction and occupation of some of the grander pueblos spanned many generations. Hundreds of people occupied them, probably a mixture of longtime residents and immigrants from outside. We’re trying to sort that out.”

“Strontium isotope analysis can separate newcomers from lifelong inhabitants of a place?”

“Yep.”

The hummingbird revved up again.

“The technique can tell you where someone lived?”

“If you have reference samples. In some circumstances, if a subject moved from one geographic region to another, Sr analysis can tell where they were born, and where they spent the last six to ten years of their life.”

The hummingbird gunned it to red-line.

“Drop back and start from the beginning.” I grabbed pen and paper. “Using no word having more than three syllables.”

“There are four stable isotopes of strontium, and one isotope, 87Sr is produced by the radioactive decay of 87Rb. The half-life’s forty-eight point eight billion years.”

“Much slower than Carbon 14.”

“Much slower than my old dog Spud.”

Spud?

“The geology of North America shows tremendous age variation,” Art sailed on, oblivious to my confusion over the dog reference. “For example, the age of the crust varies from less than a million years in Hawaii, to just over four billion years in parts of the Northwest Territories of Canada.”

“Resulting in differences in Sr values in the soil and rock of different regions.”

“Yes. But such differences are also due to variations in bedrock composition.”

“When you use the term value, do you mean the ratio of the unstable strontium to its stable counterpart?”

“Exactly. It’s the ratio of the strontium 87 isotope to the strontium 86 isotope that’s important, not the absolute level of each.”

I let him go on.

“For example, basaltic lavas, limestone, and marble all have very low Sr ratios, whereas those of sandstone, shale, and granite are commonly high. Clay minerals have some of the highest.”

“So differences in geologic age and/or bedrock composition produce variations in Sr isotope ratios in different geographic regions.”

“Precisely. But one final thing to keep in mind is that because ratios are so messy to remember, with all those decimals, we usually compare a measured Sr ratio to the average Sr ratio of the whole Earth. If the measured ratio is greater than this, it yields a positive value. If it’s less than this, it gives you a negative value.”

“What does this have to do with establishing where someone was born?”

“Strontium is an alkaline-earth metal, chemically similar to calcium.”

I made the link. “Strontium is absorbed by plants from the soil and water. Herbivores eat the plants, and on up the food chain.”

“You are what you eat.”

“So the Sr isotope composition of an organism’s bones and teeth will reflect the Sr composition of its diet during the period those body parts were forming.”

“You’ve got it.”

“My grandmother used to worry about strontium in her food.”

“Your granny wasn’t alone. The biological processing of strontium was studied extensively in the 1950s because of the potential for radioactive 90Sr ingestion due to aboveground testing of nuclear weapons.”

A light was going on.

“You’re saying strontium is incorporated into a person’s bones and teeth, much like calcium.”

“Right.”

“And calcium in the human skeleton is replaced on roughly a six-year cycle.”

“Yep.”

“So, like skeletal Ca, skeletal Sr reflects an individual’s diet over the last six years of life.”

“Six to ten,” Art said.

“But Ca levels don’t change in tooth enamel as they do in bone. Once laid down, enamel is stable.”

“And the same is true of Sr. So dental enamel continues to reflect the average dietary Sr isotope composition ingested when the tooth was formed.”

“So if someone relocated from the place in which she was living when her teeth were forming, that individual’s dental and skeletal Sr levels would differ. If she stayed put, those levels would remain similar.”

“Precisely. Enamel values suggest place of birth and early childhood. Bone values suggest place of residence during the last years of life.”

A thought stopped me in midscribble.

“Doesn’t our food come through national and international networks these days?”

“We drink local water, at least most of the time.”

“True. Tell me what you did with my specimens.”

“After removing all extraneous materials, we ground them. Then we separated out the Sr by ion-exchange chromatography, analyzed the purified Sr using thermal ionization mass spectrometry, and collected the Sr ratios by multicollector dynamic analys—”

“Art.”

“Yes?”

“What did you find?”

“One of your three saw a bit of the world.”